Conventional spear guns, such as those utilized in scuba diving, typically comprise a frame having a longitudinally elongated body mounting, at a rear end, a grip and a trigger. A shaft of a spear is mounted to a front end of the frame, the shaft being mounted slidingly on the elongated body of the frame. The spear is then launched toward a desired target, e.g., a fish, by pulling the trigger. The rear end of the shaft is usually connected to a string line for retrieval of the spear and any prey that may have been snagged by the diver.
The force for propelling the spear is provided by a pair of cables, made of an elastic material, that are stretched and hooked to the shaft of the spear when the diver loads the gun. Once the trigger is pulled, the spear is released, effecting conversion of the elastic energy accumulated by the elastic cables into kinetic energy in the spear shaft, and propelling the spear outwardly along the longitudinal axis of the gun.
The kinematic trigger mechanism that enables the diver to launch or shoot the spear generally comprises a lever hingedly mounted to the frame with a latch element at one end for engaging a notch at a proximal end of the spear's shaft when the spear is loaded on the gun so as to withstand a pulling force exerted on the shaft by the elastic cables. During loading, the proximal end of the shaft abuts a radial appendage on the lever, thereby inducing its rotation, Such rotation results not only in engagement of the latch member with the notch on the shaft, but also of the opposite end of the lever elastically within a seat in the trigger formed on the side opposite its operating arm. Disengagement of the lever from the trigger, induced when a diver's finger pulls on the operating arm, causes the shaft to be released from the latch element and the propulsive force of the elastic cables prevails.
The kinematic chains often used to operate spear guns, though useful, do not permit adjustment of the stroke of the trigger nor of the force necessary to pull it. If the stroke of the trigger is short, and the spear gun is operated by an inexperienced scuba diver who lacks a feel for the trigger, or in the event of other circumstances such as a diver's use of very thick gloves, the spear may be accidentally released. Simply put, adjustments of the trigger's stroke and/or graduating the effort required for a diver to pull the trigger has been found difficult and has yet to provide a solution that is both safe and simple.